1. Field of the Invention
The present invention relates to an ignition system for internal combustion engines which is designed so that a DC high voltage is applied to the gap of a spark plug from a time before the start of capacitive discharge at the spark plug to a predetermined time or to a predetermined crank angle after the start of the discharge, whereby controlling sustained discharge and thereby improving the ignition performance.
2. Description of the Prior Art
Recently, from the standpoint of saving of resources, the most important problem for the automobile has been to improve the fuel consumption. To accomplish this objective, there has been tendency toward increasing the compression ratio for improving the combustion efficiency and rendering the air-fuel ratio of mixtures leaner for improving the fuel comsumption. In this case, what are important with the ignition system are the dielectric breakdown voltage at the plug gap and the ignition performance. FIG. 1 shows the dielectric breakdown voltages at the plug gap with different compression ratios (fuel is propane; density is 2.5%; ignition timing is set at top dead center). The difference between the breakdown voltage at the compression ratio of 8.4 and that the compression ratio of 12.9 is about 6 KV. Also, increasing the plug gap to improve the ignition performance tends to increase the breakdown voltage, and rendering leaner the air-fuel ratio of mixtures also results in an increase in the breakdown voltage. Since the breakdown voltage increases in this way, when any of the presently available ignition systems is used, even if a high voltage is produced in the secondary winding of the ignition coil and the high voltage reaches the dielectric breakdown voltage causing a capacitive discharge, the dielectric breakdown voltage is so high as shown in FIG. 2 such that there are instances where the energy is consumed during the interval between the occurrence of the high voltage and the occurrence of capacitive discharge and the energy for sustained discharge is reduced, thus failing to cause a sustained discharge and causing an open type waveform. In this case, since capacitive energy is increased, there will be no problem if the increased capacitive energy is sufficient to ignite the mixture. However, usually the time required for the flame core in the combustion chamber to effect the flame propagation by its own ability is about 1 ms so that if the energy possessed by the flame core is small, the flame core will be extinguished during this time interval due to the cooling effect of the electrodes and the mixture. As a result, it is necessary to supply energy to the flame core until it grows up. In case of the waveform shown in FIG. 2, only the capacitive energy is supplied and the following sustained discharge energy is not supplied, making it difficult to effect the ignition. As a result, in cold starting, racing or the like where the dielectric breakdown voltage increases, misfiring tends to be caused, thus causing deterioration of the feeling, engine stalling, etc. It is known as a countermeasure, to improve the performance of an ignition coil. For instance, it is to increase the size and the number of turns of an ignition coil so a to increase the secondary high voltage of the ignition coil, or to increase the magnitude of the current at the time of deenergizing the primary winding so as increase the energy, or making the ignition coil into a closed magnetic circuit coil so as to decrease the energy loss, etc. However, these methods are subjected to limitations in view points of coil conversion efficiency, heat generation, etc., and further these methods are effective only in increasing the energy, but result in increase in wear of the spark plugs. Other methods are known in which, as disclosed in Japanese Patent Publications No. 51-39326, No. 51-106837 and No. 53 -131338, a capacitor is preliminarily charged with a high voltage so that the discharge caused at the spark plug by the voltage produced in the ignition coil is used as a trigger to discharge the charged energy of the capacitor to the spark plug. In this case, since the energy discharged by the capacitor is fixed and the energy is superposed on the energy in the secondary winding of the ignition coil, the total energy supplied to the mixture is increased and the ignition performance is improved. However, there is a disadvantage that since the capacitance energy varies in dependence on the conditions at the plug discharge gap (the density of mixture, pressure, etc.), the sustained discharge time also varies and a situation arises in which even if the flame core produced by the discharge grows up in the combustion chamber and starts the flame propagation by its own ability, the discharge is not completed as yet and wear of the spark plug electrodes is increased. Thus, the energy to be supplied must be limited to the minimum requirement.